By the time + present continuous

Hi, Izzat Hannah,

As you know, the future perfect is used to refer to a future action or state previous to another future action or state which, if expressed in the form of a subordinate clause, cannot be stated in a future tense. These sentences taken from "The grammar of the English tense system" by Renaat Declerck which combine future perfect in the main clause with future in the before-clause are defined by the author as ungrammatical:

Adverbial before-clauses, page 725, item 14.8.3:

John {will have left / *has left} before Bill {arrives / *will arrive}.
Gordon {will have visited / *has visited} the cathedral before Bryan {goes / *will go} to the town hall.

The correct sentences are:

- John will have left before Bill arrives.
- Gordon will have visited the cathedral before Bryan goes to the town hall.

Alternatively, we can use the present perfect in the subordinate clause:

- John will have left before Bill has arrived.
- Gordon will have visited the cathedral before Bryan has gone to the town hall.

The verbs in the time clauses above, arrives/has arrived, goes/has gone, indicate actions completed in the future before which something else will have occurred (John's departure, Gordon's visit).

@Izzat Hannah posted:

By the time I am finishing this sentence, you will have already started thinking about your next question.

Is this a grammatically correct sentence?

The problem with your sentence above is that:

1. if "am finishing" is interpreted as a variant of the future tense, the result is ungrammatical, as are By the time I will finish... / By the time I am going to finish... / By the time I will be finishing / By the time I will have finished;
2. if "am finishing" is interpreted as present continuous tense, the sentence does not work because the action is incomplete.

However, I think these might work because "to be in the process of" and "to be about to" indicate a fulfilled state, not an unfulfilled action as is the case with am finishing (anyway, I'd like to see what David can tell us):

- By the time I am in the process of finishing / I am about to finish this sentence, you will have already started thinking about your next question.

I would, of course, write something much simpler and safer like:

- By the time I finish / have finished this sentence, you will have already started thinking about your next question.